Fibrosis
Fibrosis is a chronic and progressive process characterized by an excessive accumulation of extracellular matrix (ECM) leading to stiffening and/or scarring of the involved tissue. It develops through complex cell, extracellular matrix, cytokine and growth factor interactions. Distinct cell types are involved such as resident mesenchymal cells (fibroblasts and myofibroblasts) and ECM-producing cells derived from epithelial and endothelial cells (through a process termed epithelial- and endothelial-mesenchymal transition), local or bone marrow-derived stem cells (fibrocytes). Myofibroblasts has long been regarded as a major cell type involved in normal wound healing, and as the key effector cell in fibrogenesis. They are highly synthetic for collagen and other ECM components, and are characterized by the de novo expression of α-smooth muscle actin (α-SMA) (reviewed in Scotton C. J. and Chambers R. C., 2007). The presence of myofibroblasts in fibrotic lesions in animal models of fibrosis correlates with the development of active fibrosis, and their persistence and localization to fibrotic foci in human disease is associated with disease progression (Kuhn C. and McDonald J. A., 1991, and Zhang et al., 1994). Myofibroblasts also exhibit an enhanced migratory phenotype (Suganuma et al. 1995) and are capable of releasing numerous pro-fibrotic mediators.
Fibrotic Diseases
Fibrotic diseases, including pulmonary fibrosis, systemic sclerosis, liver cirrhosis, cardiovascular disease, progressive kidney disease, and macular degeneration, are a leading cause of morbidity and mortality and can affect all tissues and organ systems. Fibrotic tissue remodeling can also influence cancer metastasis and accelerate chronic graft rejection in transplant recipients. Examples of primary (idiopathic) and secondary fibrotic disorders with multiple/single organ presentation are listed in Table 1. Nevertheless, despite its enormous impact on human health, there are currently no approved treatments that directly target the mechanism(s) of fibrosis.
TABLE 1Examples of primary (idiopathic) and secondary fibrotic disorders withmultiple/single organ presentation.PRIMARYSECONDARYHeartIdiopathic restrictiveCoronary artery disease/myocardial infarctioncardiomyopathyPressure-overload heart (long standing arterial hypertension,valvular disease)Infectious myocarditisAutoimmune diseasesTransplant rejectionFamilial hypertrophic cardiomyopathyArrhythmogenic right ventricular cardiomyopathyDrug-inducedPost-radiationSarcoidosisAmyloidosisKidneyIdiopathic nephroticDiabetic glomerulosclerosissyndromeIdiopathicHypertensive nephrosclerosismembranoproliferativeAutoimmune glomerular diseasesglomerulonephritisDrug-inducedPost-radiationAmyloidosisTransplant rejectionSystemicSystemic sclerosisGraft versus host diseaseSarcoidosisDrug-induced nephrogenic systemic fibrosisAmyloidosisSecondary amyloidosisPost-radiationToxic environmental exposureStorage disorders (hemochromatosis, glycogenosis,Gaucher's disease, etc.)LungsIdiopathic pulmonary fibrosisPneumoconiosisHistiocytosis XInfectious pneumonitisCryptogenic organizingTuberculosispneumoniaHypersensitive pneumonitisInherited disordersAutoimmune diseasesTransplant rejectionDrug-inducedPost-radiationSarcoidosisAmyloidosisLiverPrimary biliary cirrhosisChronic viral hepatitisPrimary sclerosingSchistosomiasischolangitisAlcoholic liver diseaseNonalcoholic fatty liver diseaseDrug-inducedToxic environmental exposureInherited metabolic disordersAutoimmune hepatitisIntestinal bypassAdapted from Vettori S, Gay S, Distler O. Role of MicroRNAs in Fibrosis, The Open Rheumatology Journal, 2012, 6, (Suppl 1: M9) 130-139.
Pulmonary Fibrosis
Lung fibrosis, also referred to as pulmonary fibrosis, is a serious medical condition that involves scarring of the lung tissue. This condition occurs when the alveoli and interstitial tissue of the lungs become inflamed and develop scars on the tissue in an attempt to repair themselves. Pulmonary fibrosis involves gradual exchange of normal lung parenchyma with fibrotic tissue (fibrous scar). The replacement of normal lung with scar tissue causes irreversible decrease in oxygen diffusion capacity. Currently, there is no cure or means by which to reverse this scarring of the lung tissue.
Pulmonary fibrosis can be caused by many conditions which includes chronic inflammatory processes (sarcoidosis, Wegener's granulomatosis), infections, environmental agents (asbestos, silica, exposure to certain gases), exposure to ionizing radiation (such as radiation therapy to treat tumors of the chest), chronic conditions (lupus), and certain medications (e.g. amiodarone, bleomycin, pingyangmycin, busulfan, methotrexate, and nitrofurantoin).
In a condition known as hypersensitivity pneumonitis, fibrosis of the lung can develop following a heightened immune reaction to inhaled organic dusts or occupational chemicals. This condition most often results from inhaling dust contaminated with bacterial, fungal, or animal products.
In some subjects, chronic pulmonary inflammation and fibrosis develop without an identifiable cause. Most of these subjects have a condition called idiopathic pulmonary fibrosis (IPF). IPF is a chronic progressive pulmonary fibrosis of unknown etiology. Prednisone is the usual treatment for IPF but it can be treated with other immunosuppressive therapies with the objective of reduction of inflammation that is the prelude to lung fibrosis. Although prednisone has a modest measurable effect on improving lung function, the scarce evidence for its long-term efficacy, as well as concerns regarding its safety, limits its use. Indeed most immunosuppressive drugs have little therapeutic effects and lung transplantation may be necessary. Unfortunately, transplants are of limited success in patients with end-stage long disease and median survival time with patients is four to six years after diagnosis. As such, there is need for novel yet efficacious treatment for IPF.
Some clinical trials are ongoing with candidate drugs that specifically address the inhibition or slowing down of fibrosis in the lungs such as interferon-γ (IFN-γ) and mycophenolate mofetil. Further examples include: pirfenidone which mechanism of action is not well defined but seems to reduce CTGF and has shown some results in clinical phase; substituted biphenyl carboxylic acids which function as lysophosphatidic acid receptor antagonists display significant antifibrotic activity in the standard pulmonary fibrosis mouse model (bleomycin-induced lung fibrosis). As such, this compound is reported to be in clinical trials for the treatment of IPF. Inhibition of protein kinase enzymes with orally active candidate drugs or treatment with orally active antioxidants provide two treatment approaches for pulmonary fibrosis: multiple receptor tyrosine kinase inhibitor (such as nintedanib) and JNK (kinase) inhibitors (such as tanzisertib). Also, drug candidates for IPF includes antioxidant N-acetylcysteine. However, to date the progress of protein kinase inhibitors and antioxidants have been questionable for the treatment of IPF due to issues of toxicity and/or efficacy. Protein kinase enzymes and associated receptors are ubiquitous amongst normal and diseased cell populations and so inhibition may result in toxicity arising in particular amongst rapidly proliferating cell populations.
Additionally, clinical trials are in progress with monoclonal antibodies that target different profibrotic proteins (cytokines (CTGF, TGF-β, MCP-1, IL-4 and IL-13), integrins (αvβ6) and enzymes (Lysyloxidase-like-2)) for the treatment of IPF. However, a number of issues are associated with the development and use of monoclonal antibodies for the treatment of IPF (which apply to other recombinant proteins) which include toxicity (including protein immunogenicity), difficulty of manufacture (batch consistency, scale-up, expense) and administration (need for refrigeration, not orally active).
Furthermore, though research trials are ongoing, there is no evidence that any medications can significantly help this condition. Lung transplantation is the only therapeutic option available in severe cases. Unfortunately, transplants are of limited success in patients with end-stage lung disease. As such, there is a need for novel yet efficacious treatments for IPF. Therefore, there is a need for novel yet conveniently administered (orally active) efficacious synthetic (readily manufactured) compounds.
Liver Fibrosis
Liver fibrosis or hepatic fibrosis is the excessive accumulation of extracellular matrix proteins (including collagen), and subsequent scarring process, that occurs in most chronic liver diseases. With time, advanced liver fibrosis results in cirrhosis of the liver. Cirrhosis is the final phase of the chronic liver disease and is generally irreversible with a poor long-term prognosis. In the advanced stage, the only option is the liver transplant. The risk of liver cancer is significant increased with cirrhosis and cirrhosis may be viewed as a premalignant condition (hepatocellular carcinoma). Indeed, cirrhosis and liver cancer are among the ten causes of death worldwide. As such, there is a need for novel yet efficacious treatment for liver fibrosis and subsequent cirrhosis of the liver. Unfortunately, few treatment options are available and most often treatment consists of addressing the causes and/or symptoms of liver cirrhosis. No treatment will cure liver fibrosis subsequent scarring and cirrhosis. Liver transplantation is the only treatment available for patients with advanced stage of fibrosis. Therefore, alternative methods that would be less intrusive are needed to cure, treat, slow the progression of, or prevent liver fibrosis.
Accumulation of fluid in the abdomen (ascites) is a common problem associated with liver cirrhosis. Treatment options include a low sodium diet, diuretics and removal of fluid by insertion of a needle into the abdominal cavity (paracentesis). Cirrhosis of the liver is caused by alcohol abuse, viral hepatitis (B, C and D), non-alcoholic fatty liver disease (NAFLD) associated with obesity, diabetes, protein malnutrition, coronary artery disease, corticosteroids, auto-immune hepatitis, inherited diseases (cystic fibrosis, alpha-1-antitrypsin deficiency, etc), primary biliary cirrhosis, drug reaction and exposure to toxins.
A limited number of clinical trials are in progress with candidate drugs that specifically address the inhibition or slowing down of fibrosis in the liver. However, these trials target specific liver disease such as NASH (Non-alcoholic Steatohepatitis). NASH refers to a combination of fatty liver (NAFLD) with inflammation and occurs in individuals who drink little or no alcohol. Cysteamine is a precursor of the potent liver antioxidant glutathione and increased in vivo production of glutathione is believed to offer improvement of NASH-related liver disease. As such, cysteamine is under evaluation in clinical trial in pediatric patients with NASH. Other antioxidants are under evaluation such as vitamin E and selenium but their effectiveness for the treatment of NASH is unknown. Also under evaluation for the treatment of NASH is the use of anti-diabetic drugs even in patients without diabetes. This approach addresses the fact that most NASH patients have insulin resistance. Once again, there is a need for novel yet conveniently administered (orally active) efficacious compound for the treatment of liver fibrosis, subsequent scarring and liver cirrhosis.
Skin Fibrosis
Skin fibrosis or dermal fibrosis is excessive scarring of the skin, and is a result of a pathologic wound healing response. There is a wide spectrum of fibrotic skin diseases: scleroderma, nephrogenic fibrosing dermopathy, mixed connective tissue disease, scleromyxedema, scleredema, and eosinophilic fasciitis. Exposure to chemicals or physical agents (mechanical trauma, burn wounds) are also potential causes of fibrotic skin disease. Dermal fibrosis may be driven by immune, autoimmune, and inflammatory mechanisms. The balance of collagen production and degradation by fibroblasts plays a critical role in the pathophysiology of fibrotic processes in the skin. Certain cytokines promote would healing and fibrosis, such as transforming growth factor-β (TGF-β) and interleukin-4 (IL-4), whereas others are antifibrotic, such as interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). Fibroblasts of normal skin are quiescent. They synthesize controlled amounts of connective tissue proteins and have low proliferative activity. Following skin injury, these cells become activated, i.e. they proliferate, express α-smooth muscle actin (α-SMA) and synthesize large amounts of connective tissue proteins. The activated cells are often called myofibroblasts.
Scar formation as part of the wound healing process and which accompanies fibrosis is particularly undesired from a cosmetic perspective during skin fibrosis, especially when the scars are formed on the face and/or other exposed parts of the body. Scleroderma refers to skin fibrosis; sclera=hard and derma-skin. However, skin fibrosis may have important health consequences, especially if it is part of systemic scleroderma. The latter refers to a connective tissue disease of auto-immune etiology. Whereas limited cutaneous scleroderma is restricted to skin on the face and on feet, diffuse cutaneous scleroderma covers more of the skin and may progress to the visceral organs.
The most popular approach for treating skin fibrosis is the use of immunosuppressive therapy. The rationale is that the auto-immune etiology is responsible for the inflammation aspect of the disease along with subsequent tissue damage and fibrosis. Studied drugs include methotrexate, mycophenolate, mofetil, cyclophosphamide and cyclosporine. Although some improvement has been observed with immunosuppressive therapy, concerns regarding drug safety along with a lack of definitive clinical data and demonstratable efficacy, remain.
There is a need to develop efficacious pharmaceutical preparation for treating skin fibrosis, fibrotic skin diseases and pathological scarring of the skin.
Renal Fibrosis
The kidney is a structurally complex organ that has evolved to perform a number of important functions: excretion of the waste products of metabolism, regulation of body water and salt, maintenance of appropriate acid balance, and secretion of a variety of hormones and autocoids. Diseases of the kidney are as complex as its structure, but their study is facilitated by dividing them by their effects on four basic morphologic components: glomeruli, tubules, interstitium, and blood vessels. Unfortunately, some disorders affect more than one structure and the anatomic interdependence of structures in the kidney implies that damage to one almost always secondarily affects the others. Thus, whatever the origin, there is a tendency for all forms of renal disease ultimately to destroy all four components of the kidney, culminating in chronic renal failure. For instance, in autoimmune diseases such as diabetes mellitus, the kidneys are prime targets to suffer tissue damage or lesions. Nephrectomy, or kidney removal, a procedure which is sometimes performed on patients with kidney cancer (e.g. renal cell carcinoma), and may negatively impact kidney function in the remaining kidney. Chemotherapy and immunosuppressive therapy are also a source of harmful effects to the kidneys. All these kidney injuries result in most of the cases in renal fibrosis. The term “renal fibrosis” means excessive proliferation of cells, hardening tissue and scarring. Renal fibrosis can also result from dialysis following kidney failure and catheter placement, e.g., peritoneal and vascular access fibrosis. Renal fibrosis may also result from a nephropathy such as glomerular diseases (e.g. glomerulosclerosis, glomerulonephritis), chronic renal insufficiency, acute kidney injury, end stage renal disease and renal failure. Regardless of etiology, all patients with chronic renal disease show a progressive decline in renal function with time. Fibrosis, so-called scarring, is a key cause of this pathophysiology. Fibrosis involves an excess accumulation of extracellular matrix (primarily composed of collagen) and usually results in loss of function when normal tissue is replaced with scar tissue. The process is largely irreversible, inevitably leading to end-stage renal failure, a condition that requires life-long dialysis or renal transplantation. Recent major advances have led to a much better understanding of renal fibrosis (or renal tubulointerstitial fibrosis), many problems remain. Little is known about why some wounds heal and others scar and little about how many putative antifibrotic agents work.
There is a need to develop efficacious pharmaceutical preparation for treating renal fibrosis.
Cardiac Fibrosis
Cardiac fibrosis, a hallmark of heart disease, is thought to contribute to sudden cardiac death, ventricular tachyarrhythmia, left ventricular (LV) dysfunction, and heart failure. Cardiac fibrosis is characterized by a disproportionate accumulation of fibrillated collagen that occurs after myocyte death, inflammation, enhanced workload, hypertrophy, and stimulation by a number of hormones, cytokines, and growth factors.
Cardiac fibrosis may also refer to an abnormal thickening of the heart valves due to inappropriate proliferation of cardiac fibroblasts but more commonly refers to the proliferation of fibroblasts in the cardiac muscle. Fibrocyte cells normally secrete collagen, and function to provide structural support for the heart. When over-activated this process causes thickening and fibrosis of the valve, with white tissue building up primarily on the tricuspid valve, but also occurring on the pulmonary valve. The thickening and loss of flexibility eventually may lead to valvular dysfunction and right-sided heart failure.
The most obvious treatment for cardiac valve fibrosis or fibrosis in other locations, consists of stopping the stimulatory drug or production of serotonin. Surgical tricuspid valve replacement for severe stenosis (blockage of blood flow) has been necessary in some patients. Also, a compound found in red wine, resveratrol, has been found to slow the development of cardiac fibrosis. [Olson et al. (2005) “Inhibition of cardiac fibroblast proliferation and myofibroblast differentiation by resveratrol”. American journal of physiology. Heart and circulatory physiology 288 (3): H1131-8; Aubin, et al. (2008) “Female rats fed a high-fat diet were associated with vascular dysfunction and cardiac fibrosis in the absence of overt obesity and hyperlipidemia: Therapeutic potential of resveratrol”. The Journal of Pharmacology and Experimental Therapeutics 325 (3): 961-8. More sophisticated approaches of countering cardiac fibrosis like microRNA inhibition (miR-21, for example) are being tested in animal models.
No medication is on the market to prevent or treat cardiac fibrosis and there is a need to develop efficacious pharmaceutical preparation.
Pancreatic Fibrosis
Chronic pancreatitis (CP) is a progressive inflammatory disease of the pancreas, characterized by irreversible morphologic changes and gradual fibrotic replacement of the gland. Loss of exocrine and endocrine function results from parenchymal fibrosis. The primary symptoms of CP are abdominal pain and maldigestion. Grossly, the pancreas may be enlarged or atrophic, with or without cysts or calcifications or tumors. The ducts may be dilated, irregular, or strictured. Essential pathologic features include irregular and patchy loss of acinar tissue, chronic inflammation, ductal changes, and fibrosis. These gross changes are end-manifestations of complex pathogenic mechanisms that are associated with gene mutations (including but not limited to cystic fibrosis, cationic trypsinogen gene, CFTR gene mutations in idiopathic acute and chronic pancreatitis, the pancreatic secretory trypsin inhibitor gene, the chymotrypsinogen C gene and the calcium sensing receptor gene, alpha-1 antitrypsine deficiency), metabolic (alcoholic, tobacco smoking, hypercalcemia, hyperlipidemia, chronic renal failure), environmental factors (nutritional factors such as micronutrient dificiencies (zinc, copper and selenium; also by postadiation exposure), obstructive (tumors), ischemic (vascular diseases), and autoimmune or associated with primary sclerosing cholangitis, Sjögren's syndrome, primary biliary disorder and type 1 diabetes mellitus. Because of diagnostic and therapeutic challenges, an interdisciplinary management strategy is required.
Macular Degeneration
Most diseases that cause catastrophic loss of vision (e.g. macular degeneration) do so as a result of abnormal angiogenesis and wound healing, often in response to tissue ischemia or inflammation. Disruption of the highly ordered tissue architecture in the eye caused by vascular leakage, hemorrhage, and concomitant fibrosis can lead to mechanical disruption of the visual axis and/or biological malfunctioning. The CNS is highly specialized in many ways, including the types of inflammatory and wound-healing cells present. Since the retina is part of the CNS, its response to injury utilizes mechanisms very similar to those observed in the rest of the brain; this is true not only for the wound-healing response but also for utilization of migratory cues functional during development of the neuronal and vascular component of this highly organized tissue (Friedlander M.; Fibrosis and diseases of the eye, J. Clin. Invest. 2007). As discussed below, the response of the anterior segment of the eye to wound healing more closely resembles the response of non-CNS tissues than do such events in the posterior segment or the eye. Therefore, I refer to such wound-healing events in the anterior segment as fibrosis, whereas comparable events in the retina are referred to as gliosis. Although such distinction is somewhat artificial, it does serve to differentiate between the fibroblasts and glial cells that effect the wound-healing and scar-formation events. An increased understanding of inflammation, wound healing, and angiogenesis has led to the development of drugs effective in modulating these biological processes and, in certain circumstances, the preservation of vision.
Unfortunately, such pharmacological interventions often are too little, too late, and progression of vision loss frequently occurs.
There is need to prevent or treat each fibrotic disease with a safe and efficacious drug.